Digital recording of data is now commonly used in security and surveillance applications. Typically, a Digital Video Recorder (DVR) records content, such as video, audio and often other data onto disks or other recording media. The recording is either done continuously or is triggered by specific events.
A DVR has a finite amount of recording space on which the recoded data is stored. Accordingly, the DVR can record for a finite amount of time. To allow for continuous use, the DVR storage space must be made available. To free storage space, the DVR periodically deletes data to make room for new recordings. Video data, in particular, can consume a great deal of space, and must be regularly deleted. Deletion of data can cause problems if the data is still needed by a user. To reduce the frequency of data deletion, and thus allow more data to be recorded, the DVR can capture data at a lower resolution, or can use higher degrees of lossy compression. Both use of a lower resolution and higher lossy degrees of compression result in a reduced quality video stream, which can be problematic if a higher quality video stream is required to properly analyze the data. It is well known, that at most times, only a reduced quality data stream is needed, with a higher quality data stream required only to capture information about a specific incident or event. One problem is that the higher quality image is often required before the event occurs so that an after-the-fact review can make use of information leading up to and following the event. A number of methods have been used to ensure that data is available when needed, and is available at the required quality. The different methods are not mutually exclusive, and are often combined.
The easiest method is to provide more storage space by installing large disk drives, or multiple drives. While this method is effective, it can also be costly, in some cases dominating the cost of the DVR.
Another approach is to record selectively. In this case, the DVR records only at “significant” times, such as in response to specific events (e.g. motion or alarms), or at previously scheduled times in the day. Because the DVR is not continuously recording there is a reduction in the amount of data captured. Thus, the DVR can operate for longer periods before reaching capacity. The disadvantage of this method is that the rules must be set up in advance. If the DVR is configured to record for too short a time after an event, important information may not be recorded. Alternately, if the DVR is configured to record for too long a time, the storage capacity issue may not be adequately addressed. As noted above, this also fails to provide any information leading up to the event. Yet another approach is to adjust the recording parameters. By reducing the quality of the recorded video, by reducing its frame rate or by reducing the size of each frame, the volume of data can be reduced significantly. This has the obvious drawback that, while it might significantly reduce data rates and improve retention time, it might render the resulting data useless for its intended purpose. For example, video of a bank robbery may be of little use if the quality or frame size is not sufficient to recognize a person's face.
It is, therefore, desirable to provide a method and system for storing data that permits storage and retrieval of high quality recordings, while permitting storage space to be freed up when necessary.